In a welded structure forming a building structure, it goes without saying that the weld joint characteristics have to be superior. In recent years, possession of superior tensile strength at a high temperature, the characteristic of so-called “fire-resistant steel” (fire-resistant performance), has become sought.
This is a characteristic decided on by the Japanese Ministry of Land, Infrastructure, Transportation, and Tourism based on the “New Fire-Resistant Design Law” enabling use of steel materials without fire-resistant coverings considering environmental issues and is based on the performance based on MLIT Notification No. 333 (2004).
Here, “fire-resistant performance” is the performance required for enabling a steel material to continue to exhibit the necessary strength for a certain time when a steel material is exposed to a fire in an uncovered state and facilitating the escape of residents by preventing the building structure from collapsing during that time.
When a steel material is not provided with a fire resistant covering, various sizes of fires and ambient temperatures at the time of fires may be envisioned, so the high temperature strength required for a steel material supporting the strength of a structure is required to be as high as possible.
Steel materials provided with such fire-resistant performance have long been the subject of R&D in all different fields.
For example, disclosures of inventions relating to steel materials containing Mo and high in high temperature strength may be found in (a) Japanese Patent Publication (A) No. 2001-294984, (b) Japanese Patent Publication (A) No. 10-096024, and (c) Japanese Patent Publication (A) No. 2002-115022.
The arts disclosed in these PLT's a to c all relate to materials raised in high temperature strength by precipitation strengthening by Mo carbides or precipitation strengthening by other carbides plus texture strengthening so as to raise the high temperature strength.
On the other hand, due to the pinch in supply and demand of various types of alloy elements, industrially speaking the addition of Mo ends up raising the costs of the steel materials. Due to this reason, disclosures of arts employing other alloy designs have been seen.
In particular, the example of the invention described in (d) Japanese Patent Publication (A) No. 07-286233 adding B to improve the quenchability so as to secure high temperature strength aiming at a 600° C. or so temperature, the example of the invention described in (e) Japanese Patent No. 3635208 adding γ-phase stabilizing elements of Cu, Mn, etc., etc. may be mentioned.
However, when unintentionally adding γ-phase stabilizing elements such as in the PLT e or adding B for the purpose of suppressing formation and growth of nuclei from the grain boundaries to form a low temperature transformed structure such as in the PLT d, there is the problem that remarkable embrittlement occurs when the grain boundaries of the steel material are exposed to a high temperature (phenomenon of ductility being impaired at the time of high temperature deformation, called “reheat embrittlement”).
According to research of the inventors, in such a steel material, no matter how high the high temperature strength, there is almost no high temperature deformation ability, so it became clear that when designing deformation of the structure so to be borne concentratedly at the weld joints or when breakage occurs, mainly the HAZ (heat affected zone) and the grain boundaries at the HAZ side near the borders with the weld metal cannot keep up with deformation at the time of a high temperature of a fire and grain boundary breakage occurs in some cases.
The above-mentioned embrittlement phenomenon (reheat embrittlement phenomenon) mainly includes cases of embrittlement due to grain boundary precipitation and cases of segregation causing only the grain boundaries to drop in transformation point, the strength of the grain boundary parts remarkably dropping and local deformation occurring, and as a result breakage such as peeling from the grain boundaries occurring. It changes in various ways depending on the chemical ingredients of the steel materials. This was clarified by research of the inventors.
In the above way, when a steel material is exposed to a high temperature and is held at a temperature near 600° C. at the time of a fire, embrittlement of the grain boundaries occurring near the weld metal of an HAZ (drop in ductility at time of high temperature deformation) sometimes may lead to difficult-to-predict major deformation occurring along with unstable breakage modes at the weld joint even when the base material part of a steel structure raised in high temperature strength is sound.
For this reason, design of the structure becomes difficult. As a result, even if a steel material has sufficient high temperature strength, the fire resistant structure will clearly become an unsuitable structure.
None of the conventional fire-resistant steels described in the above PLT's a to c were designed in alloys considering the grain boundary embrittlement at the time of reheating the HAZ (that is, at the time of fire). They only give findings regarding the alloy design when focusing only on high temperature strength, in particular high temperature tensile strength.
Such conventional fire-resistant steels have Mo or B added for the purpose of improving the high temperature strength. On this point, they are based on elements with high abilities to form Mo carbides or B nitrides precipitating at the grain boundaries at 600° C. temperature.
On the other hand, the above-mentioned reheat embrittlement phenomenon is not manifested simply by just precipitation embrittlement. This phenomenon was first clarified as a result of the research of the inventors and is a new problem to be solved.
In the past, in the field of heat resistant steel, it was known that the reheat embrittlement was lightened by adding Cr to 2% or more and, further, that with an amount of addition of 0.5% or less, reheat embrittlement did not easily occur.
If gradually adding Cr to a steel material not containing Cr and the amount of addition exceeds 0.5%, the structure easily transforms to bainite and the material strength is improved. This is a result of improvement of the quenchability. At the same time, however, a bainite structure has old γ-grain boundaries clearly remaining at it, so at the old γ-grain boundaries, embrittlement easily is manifested and reheat embrittlement becomes easier.
On the other hand, if adding 2% or more of Cr, ordinary carbides, for example, cementite, become unstable, Cr23C6 carbides are formed, and other carbides, for example, Mo2C, are similarly robbed of carbon by Cr, and coarsening becomes more difficult at the grain boundaries. Due to this, it had been thought that grain boundary embrittlement could be prevented, but on the other hand Cr23C6 carbides also easily precipitated at the grain boundaries.
In this way, while many hypotheses like the above have been proposed, no final interpretation regarding the relationship between the amount of addition of Cr and reheat embrittlement has yet been established.
Under such current circumstances, the inventors etc. engaged in intensive research. As a result, they discovered that the reheat embrittlement phenomenon is related to the transformation point of the steel material.
That is, the addition of Cr has the effect of raising the transformation point of the steel material and simultaneously consuming the solid solution C to raise the transformation point. On the other hand, adding larger amounts of Ni and Mn known as γ-stabilizing elements lowers the transformation point. For this reason, it was discovered that when carbon etc. concentrates at the grain boundaries, at the high temperature region covered by the present invention, that is, at a 600° C. temperature, the transformation point and the high temperature yield strength evaluation temperature approach each other, part of the grain boundaries undergo α→γ transformation to be already changed in phase, numerous dislocations are lost from the structure at the time of change of arrangement of the atoms, and the strength remarkably falls, whereby breakage occurs from the grain boundaries.
As a result, raising the transformation point of the steel material is essential. Simultaneously, addition of a large amount of elements high in affinity with carbon and easily precipitating at the grain boundaries is effective in the point of raising the high temperature strength, but simultaneously the sensitivity of the HAZ to reheat embrittlement ends up being raised and the design of the structure is made more difficult. This became clear as a new problem.
Still further, in recent years, buildings have been built larger in size and higher in number of stories for the purpose of efficient utilization of land, but this larger size of structures invites an increase in size of the building materials, that is, steel plates, steel shapes, or steel pipes. To improve the efficiency of production of these steel products or improve the efficiency of assembly, the heat input at the time of welding tends to be made higher. For this reason, to obtain sufficient earthquake resistance even when the weld heat input is high, it was necessary to obtain sufficiently high weld zone toughness.